A composite material is a physical combination of two or more materials, both of which are usually visible to the naked eye. A very early example is the use of straw, in biblical times, to reinforce mud bricks for building. More recent examples are the use of steel bars to reinforce concrete and the use of glass fiber to reinforce polyester resin in the making of boats and also the use of carbon fiber to reinforce various resins for aircraft construction.
The use of composite materials in military fighter aircraft has increased very rapidly in the last 20 years. For instance, in the F14, which is a 20-year-old design, 0.8% of the total structural weight of the aircraft was made from composite. In the F15, 1.6% was composite; in the F16, 2.5%; in the F18, 9.5%; and in the ATF, which is now being designed, 50% of the aircraft structural weight is expected to be composite. In large transport aircraft in 1985, 3% of the aircraft's structural weight was composite and, by 1995, this percentage is expected to increase to 65% unless the development of aluminum lithium alloys is successful, in which case, the 65% figure will be reduced somewhat. All the above Figures are based upon estimates by industry sources and while that may not be precisely correct they will serve to indicate a trend.
It had been hoped that these composite structures could be held together by adhesives, but this has not been found practical. In conventional aluminum aircraft, approximately 25 fasteners per square foot of external surface area are used. In composite aircraft, it is still necessary to use approximatley 12 fasteners per square foot of outside surface area. These figures represent one industry's estimate.
Up to the present time, specially-shaped and modified metal fasteners have been used in composite structures. These fasteners have been reasonably satisfactory, but they are now seen as being, in many cases, unnecessarily strong, unnecessarily heavy, unnecessarily expensive and undesirably reflective to radar detection devices. Thus, a need exists for a non-metallic fastener which is sufficiently strong but yet lighter and less expensive than metal. The particular type of fastener desired is an aircraft threaded shear pin and collar, more commonly referred to as a "nuts" and "bolts."
Plastic materials have been used for nuts and bolts, but those unreinforced materials that have been produced in the past have not been strong enough for use as aircraft fasteners. Typically, such non-aircraft fasteners have possessed shear strengths of around 5-10 KSI (thousand sounds per square inch). Recently, injection-molded fiber reinforced nuts and bolts have been produced having shear strengths in the range of 9-18 KSI and tensions strengths around 4-14 KSI. These are satisfactory for aircraft use in some applications. Also, compression-molded fasteners reinforced with glass or carbon filament fiber have been produced with shear strengths in the range of 13-40 KSI, and these may find considerable use in aircraft. However, the tension strength of these compression-molded fasteners is low due to the difficulty of producing screw threads containing sufficient reinforcing fibers which are oriented in the direction needed to provide the tension strength. For this reason, the threads strip from the main shaft of the bolt at tension values in the range of 4-19 KSI.
Accordingly, a need still exists for improved non-metallic fasteners having the necessary strength characteristics.